Understanding and Resolving Rust Error E0264

Introduction to Rust Error E0264

Rust Error E0264 occurs when an unknown external lang item is used in the code. This error typically arises from using a lang item that is not recognized by the Rust compiler. In this article, we will discuss what lang items are, and provide examples of erroneous and correct usage to help you resolve Rust Error E0264.

What are lang items?

Language items (lang items) are special functions, types, and traits that have specific meanings in the Rust compiler. They are used by the compiler itself to understand the structure and semantics of the code. Lang items are defined using the #[lang] attribute.

Erroneous Code Example

Here is the erroneous code example from the official Rust documentation that triggers Rust Error E0264:

#![allow(unused)]
#![feature(lang_items)]

fn main() {
    extern "C" {
        #[lang = "cake"] // error: unknown external lang item: `cake`
        fn cake();
    }
}

In this code, the lang item 'cake' is not recognized by the Rust compiler, which causes the error. We need to use a valid lang item instead.

Correct Usage

Below is a corrected version of the code that uses a valid lang item. The lang item 'panic_impl' is known to the Rust compiler:

#![allow(unused)]
#![feature(lang_items)]

fn main() {
    extern "C" {
        #[lang = "panic_impl"] // ok!
        fn cake();
    }
}

Available External Lang Items

To avoid Rust Error E0264, you can use the available lang items listed in the Rust source file src/librustc_middle/middle/weak_lang_items.rs. These items are recognized by the Rust compiler and can be used without triggering the error.

Conclusion

Rust Error E0264 occurs when an unknown external lang item is used in the code. By using valid lang items and referencing the Rust compiler's source code, you can resolve this error and ensure your code compiles correctly. Remember that lang items have special meanings in the Rust compiler, and it's essential to use them correctly in your code.